1. Field of Invention
The present invention relates to wavelength converted semiconductor light emitting devices.
2. Description of Related Art
Light emitting diodes (LEDs) are well-known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. Traditionally, the most efficient LEDs emit light having a peak wavelength in the red region of the light spectrum, i.e., red light. However, III-nitride LEDs have been developed that can efficiently emit light having a peak wavelength in the UV to green region of the spectrum. III-nitride LEDs can provide significantly brighter output light than traditional LEDs.
In addition, since light from III-nitride devices generally has a shorter wavelength than red light, the light generated by the III-nitride LEDs can be readily converted to produce light having a longer wavelength. It is well known in the art that light having a first peak wavelength (the “primary light”) can be converted into light having a longer peak wavelength (the “secondary light”) using a process known as luminescence/fluorescence. The fluorescent process involves absorbing the primary light by a wavelength-converting material such as a phosphor, exciting the luminescent centers of the phosphor material, which emit the secondary light. The peak wavelength of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a particular peak wavelength.
With reference to FIG. 1, a prior art phosphor LED 10 described in U.S. Pat. No. 6,351,069 is shown. The LED 10 includes a III-nitride die 12 that generates blue primary light when activated. The III-nitride die 12 is positioned on a reflector cup lead frame 14 and is electrically coupled to leads 16 and 18. The leads 16 and 18 conduct electrical power to the III-nitride die 12. The III-nitride die 12 is covered by a layer 20, often a transparent resin, that includes wavelength-converting material 22. The type of wavelength-converting material utilized to form the layer 20 can vary, depending upon the desired spectral distribution of the secondary light that will be generated by the fluorescent material 22. The III-nitride die 12 and the fluorescent layer 20 are encapsulated by a lens 24. The lens 24 is typically made of a transparent epoxy or silicone.
In operation, electrical power is supplied to the III-nitride die 12 to activate the die. When activated, die 12 emits the primary light away from the top surface of the die. A portion of the emitted primary light is absorbed by the wavelength-converting material 22 in the layer 20. The wavelength-converting material 22 then emits secondary light, i.e., the converted light having a longer peak wavelength, in response to absorption of the primary light. The remaining unabsorbed portion of the emitted primary light is transmitted through the wavelength-converting layer, along with the secondary light. The lens 24 directs the unabsorbed primary light and the secondary light in a general direction indicated by arrow 26 as output light. Thus, the output light is a composite light that is composed of the primary light emitted from die 12 and the secondary light emitted from the wavelength-converting layer 20. The wavelength-converting material may also be configured such that very little or none of the primary light escapes the device, as in the case of a die that emits UV primary light combined with one or more wavelength-converting materials that emit visible secondary light.
As III-nitride LEDs are operated at higher power and higher temperature, the transparency of the organic encapsulants used in layer 20 tend to degrade, undesirably reducing the light extraction efficiency of the device and potentially undesirably altering the appearance of the light emitted from the device. Several alternative configurations of wavelength-converting materials have been proposed, such as growth of LED devices on single crystal luminescent substrates as described in U.S. Pat. No. 6,630,691, thin film phosphor layers as described in U.S. Pat. No. 6,696,703, and conformal layers deposited by electrophoretic deposition as described in U.S. Pat. No. 6,576,488 or stenciling as described in U.S. Pat. No. 6,650,044. However, one major disadvantage of prior solutions is the optical heterogeneity of the phosphor/encapsulant system, which leads to scattering, potentially causing losses in conversion efficiency.